Early this year, Steven Cook was a lawyer representing chemical companies suing to block a new rule that would force them to clean up pollution from “forever chemicals,” which are linked to low birthrates and cancer.

Now Mr. Cook is in a senior role at the Environmental Protection Agency, where he has proposed scrapping the same rule his former clients were challenging in court. His effort could shift cleanup costs away from polluters and onto taxpayers, according to internal E.P.A. documents reviewed by The New York Times.

Last month Mr. Cook met with industry groups that are still challenging the rule in court. By the next business day after the meeting, the E.P.A. office that oversees toxic cleanups had reversed its internal recommendation on the rule, the documents show, to advise repealing instead of upholding it.

The change was evident in a presentation being prepared for Lee Zeldin, the E.P.A. administrator. The document contained edits saying that the office recommended repealing the rule and that its “cons outweigh pros.” Previously, the document had recommended keeping the rule in place and that its “pros outweigh cons.”

The rule in its current form could require the chemicals industry and others to pay billions of dollars in cleanup costs for chemicals known as PFAS, or forever chemicals. The reversal in the E.P.A.’s internal guidance, and Mr. Cook’s role in the policy change, has not been previously reported.

“It’s outrageous,” said Tracey Woodruff, a researcher at the University of California San Francisco who studies environmental health, particularly the effects of chemical exposures on pregnant mothers and their babies. “If they overturn this, it would leave the public responsible for cleaning up, not the companies that knowingly polluted the land.”